An image forming apparatus for electrophotographically forming an image on a sheet includes a photoreceptor drum, a charging device, an exposure device, a developing device, a transfer device, a fixing device, a cleaning device, and a charge-removing device. In the image forming apparatus, the charging device uniformly charges a surface of the photoreceptor drum with the photoreceptor drum driven to rotate. The photoreceptor drum thus charged is irradiated with laser light by the exposure device, with the result that an electrostatic latent image is formed on the photoreceptor drum.
Then, the developing device develops, with toner, the electrostatic latent image formed on the photoreceptor drum, with the result that a toner image is formed on the photoreceptor drum as a visible image. The toner image formed on the photoreceptor drum is transferred onto a sheet (transfer receiving material, recording material, paper sheet) by the transfer device. The toner image transferred onto the sheet is pressed and heated by the fixing device so as to be fixed onto the sheet, with the result that an image is formed on the sheet.
Meanwhile, the toner remaining on the photoreceptor drum without being transferred to the sheet is removed from the photoreceptor drum by the cleaning device, and then collected in a predetermined collecting section. After the cleaning, the charge remaining on the photoreceptor drum is removed by the charge-removing device.
As a developer that is stored in the developing device to develop the electrostatic latent image formed on the photoreceptor drum, a one-component developer composed solely of toner or a two-component developer composed of toner and carrier is used.
A developing device in which the one-component developer is stored does not use carrier, and therefore does not need to include a stirring mechanism for uniformly mixing toner and carrier. This brings about an advantage of making it possible to design a developing device simply, but brings about a disadvantage of making it difficult to stabilize the amount of charge of toner.
A developing device in which the two-component developer is stored needs to include a stirring mechanism for uniformly mixing toner and carrier. This brings about a disadvantage of complicating the structure of a developing device, but brings about excellent stability in the amount of charge of toner and excellent compatibility with a high-speed machine. Therefore, the two-component developer is heavily used in a high-speed image forming apparatus and a color image forming apparatus.
In order to form a high-quality image, an image forming apparatus in which the two-component developer is used needs to maintain, at an appropriate value of density (target value), the toner density of a developer stored in a developer tank of a developing device. Accordingly, the developing device detects the magnetic permeability of the developer, for example, as an index of toner density of the developer, and is supplied with toner from a toner bottle when the toner density falls short of the target value, i.e., when the value of the detected magnetic permeability exceeds a reference value of magnetic permeability serving as a reference value for determining whether to supply toner. With this, the toner density of the two-component developer stored in the developer tank is held in the vicinity of the target value.
Further, in the image forming apparatus in which the two-component developer is used, the two-component developer deteriorates when used over time. Therefore, the two-component developer stored in the developing device needs to be replaced with a brand-new two-component developer regularly or irregularly.
It should be noted here that the toner density of the brand-new two-component developer to be newly stored in the image forming apparatus has been adjusted to density suitable for the development characteristics of the image forming apparatus (i.e., to the target value). However, the brand-new two-component developer often differs in properties from the running two-component developer that has been used in the image forming apparatus to some extent. Therefore, there have been cases where the first to several-thousandth images formed after replacement of two-component developers suffer from instability in image density.
In order to suppress a difference in properties of two-component developer between before and after replacement, Patent Document 1 discloses a technique by which the toner density of a brand-new developer is set to be lower than an appropriate level of toner density (i.e., than the target value) for use in an image forming apparatus and the toner density of the two-component developer is raised (i.e., corrected) to the target value by stirring the developer with the developer supplied with toner from a toner bottle immediately after replacement.
(Patent Document 1)
    Japanese Unexamined Patent Application Publication No. 95536/1999 (Tokukaihei 11-95536; published on Apr. 9, 1999)
However, in order to stabilize the quality of images that are formed immediately after replacement of two-component developers, it is insufficient to suppress a difference in properties of two-component developer between before and after replacement. The following gives a reason for this.
A large number of mass-produced image forming apparatuses inevitably vary from one another in dimensional accuracy such as the distance between a doctor blade and a developing roller (such a distance being hereinafter referred to as “DG”) and the distance between a photoreceptor drum and a developing roller (such a distance being hereinafter referred to as “DSD”) (manufacturing errors). As a result of their diligent study, the inventors have found that the variations in DG and DSD cause instability in density of the first to several-thousandth images formed after replacement of two-component developers. It should be noted here that the finding that the variations in DG and DSD cause the instability has been brought to light by the inventors and was not obvious at the time of the present invention. The following further details the finding that the variations in DG and DSD cause the instability. It should also be noted that the following description has been brought to light by the inventors and was not obvious at the time of the present invention.
The optimum value of toner density of a two-component developer from the first to several-thousandth images after replacement of two-component developers slightly varies depending on differences in DG and DSD. That is, immediately after replacement of two-component developers, the optimum value of toner density of a two-component developer slightly shifts from the target value, depending on the values of DG and DSD.
FIG. 6 is a graph showing a relationship between the developing potential and image density ID of an image formed in cases where the toner density of a two-component developer is appropriate. FIG. 7 is a graph showing a relationship between the developing potential and image density ID of an image formed in cases where the toner density of a two-component developer is higher than the optimum value. FIG. 8 is a graph showing a relationship between the developing potential and image density ID of an image formed in cases where the toner density of a two-component developer is lower than the optimum value. The “developing potential” here means the absolute value of a difference between the surface potential of a photoreceptor drum and the potential of a developing roller. Further, FIGS. 6 through 8 assume that the potential of a developing roller is −500 V.
In cases where the toner density of a two-component developer is higher than the optimum value as shown in FIG. 7, the gamma value (the slope of a function of developing potential and image density ID) becomes so high that the reproducibility of halftone density (ID=0.5 to 0.8) deteriorates, although the density of a solid image can be improved. In the case of a wide DG or a narrow DSD, the toner density of a two-component developer tends to be higher than the optimum value. Further, in cases where the toner density of a two-component developer is lower than the optimum value as shown in FIG. 8, the image density ID of a high-density side becomes so low that it becomes difficult to form a high-density image.
As a result of the foregoing study, in order to stabilize image quality even immediately after replacement of two-component developers, it is necessary not only to correct the toner density of a two-component developer after replacement so as to suppress a difference in properties of two-component developer between before and after replacement, but also to correct the toner density of a two-component developer after replacement so that the toner density takes on a value suitable for the values of DG and DSD.